Characterisation of temperature-dependent phase transitions in 2,2-trimethylenedioxy-4,4,6,6-tetrachlorocyclotriphosphazene, N3P3Cl4[O(CH2)3O]

Abstract

Background: The crystal structure of 2,2-trimethylenedioxy-4,4,6,6-tetrachlorocyclotriphosphazene has been determined at 120, 274 and 293 K. The result at 293 K confirms the room temperature Cmc2(1) structure, but at the lower temperatures the space group is Pna2(1). Nevertheless the basic structure remains the same, with only small displacements of the atoms, amounting to an average of 25 pm between 120 and 293 K. Results: X-ray diffraction and DSC results indicate that the phase transition takes place in two steps between 274-293 K and provides an understanding of previous NQR results. In the intermediate temperature range the molecules are displaced from their room temperature positions in such a way as to give an average structure with Cmc2(1) symmetry. Conclusion: The overall phase transition is consistent with the occurrence of a soft lattice mode at room temperature in which a large displacement of the molecule in the x-direction is coupled with a flexing motion about an axis defined by the nitrogen atoms in the N1 and N3 positions.